Re: Replication questions - read-only and temporary read/write slaves

On Fri, Jan 5, 2018 at 3:58 AM, Tiffany Thang <tiffanythang(at)gmail(dot)com> wrote:
> Thanks for your input. What I meant to say was rolling back all the changes.
> I was hoping for a way to temporary open the read-only standby in r/w for
> testing purpose and then rollback all the changes made during the test
> without having to re-create the standby from scratch.

There is no backend-side feature that allows undo actions, Postgres
only supports redo. Recycling an older standby is the speciality of
pg_rewind, which supports the possibility of backward timeline lookups
from 9.6. So you could emulate the same behavior as Oracle by:
1) Promoting the standby where you want the tests to happen.
2) Run your r/w load on it.
3) Stop the standby.
4) Rewind the standby using pg_rewind, so as it is able to join back
the cluster. This needs a new recovery.conf of course. pg_rewind also
needs to find in the standby's pg_xlog all the WAL segments from the
point where WAL has forked (when the standby has been promoted), up to
the point where you run your r/w tests. This can be tricked with
wal_keep_segments, with a replication slot or with larger values of
checkpoint_timeout and max_wal_size, or by even copying segments from
an archive before running the rewind. In all cases be careful of bloat
in the partition of pg_xlog.
--
Michael